This invention relates to a cathode device for a cathode-ray tube, and more particularly to a direct-heating type cathode device for a cathode-ray tube.
A cathode-ray tube such as, for example, a color picture tube is provided with a cathode adapted to emit three electron beams for causing the phosphor dots of the three primary colors, red, blue and green to luminesce. This cathode includes a metal base mainly containing nickel and a coating of an oxide of barium, for example, deposited to the metal base. The cathode is made to operate by being heated to about 800.degree. C. By the manner in which the cathode heating is effected, cathodes of this type are classified into indirect-heating cathodes which are provided with a separate heater and direct-heating cathodes which are heated by the electric current supplied directly thereto.
In the conventional cathode-ray tube which is provided with a direct-heating cathode, the cathode disposed inside the bulb of the cathode-ray tube is heated with an external power source through the medium of a transformer disposed outside the bulb. To be specific, terminals of the cathode are connected to pins of the cathode-ray tube base and a low-voltage secondary coil of the transformer is connected to pins of a socket adapted to be fitted on the base pins, so as to form a cathode device comprising the cathode and the transformer. In this arrangement, the cathode is heated by feeding thereto voltage of the external power source applied across a high-voltage primary coil of the transformer. Owing to its features such as low power consumption and rapid operation, the cathode-ray tube provided with such a direct-heating cathode device as described above has come to find widespread use.
Incidentally, since the direct-heating cathode is heated by direct supply of an electric current to the cathode itself, the voltage for the heating is lower and the amount of current for the heating is higher than are required in the case of the indirect-heating cathode. For example, the heating voltage and current for each indirect-heating cathode are 6.3 V and 0.25 A, whereas those for each direct-heating cathode are 0.5 V and 1.3 A. Since the heating voltage for the direct-heating cathode is conspicuously low as indicated above, foreign matters and/or oxide film lying between the base pin of the cathode-ray tube and the socket pin remain intact under the application of the voltage for the heating, so that the contact resistance between the base pin and the socket pin is increased, resulting in shortage of heating current and unstable heating.
In the conventional color picture tube which is provided with a cathode device incorporating an external transformer, when operating the cathode, each cathode for three primary color electron beam emission is separately heated and the number of input terminals to the color picture tube will be greater than in the color picture tube using an indirect-heating cathode. While a total of five terminals, i.e. two for the heater and three for the cathodes, suffices for the indirect-heating type, a total of six terminals, i.e. two for each of the three cathodes, is required for the direct-heating type.